Recently, attention has been paid to a heat transfer recording system in which an image-receiving sheet is laid on the inked surface of a sheet of paper or a plastic film coated on the surface with a heat-meltable dye (hereinafter referred to as "transferring sheet"), said transferring sheet being heated by means of a thermal head, thereby the ink layer of said transferring sheet being fluidized and transferred to said image-receiving sheet, then said two sheets being released one from the other, thus an image being obtained on said image-receiving sheet. For example, U.S. Pat. No. 3,983,279 discloses coating a base sheet with materials, such as a combination of a metallic salt of fatty acid and a phenol, which develop color by the reaction of two components, either or both of said materials being melted by means of a thermal head and transferred to an image-receiving sheet, thereby a color developed image being obtained. U.S. Pat. No. 4,474,859 discloses that the image-receiving layer of an image receiving sheet, which is used in combination with a transferring sheet coated on the surface with a heat sublimatable dye, comprises a resin such as saturated polyester resin and polyvinyl pyrrolidone, and if necessary, fillers such as calcium carbonate.
The image-receiving sheet used in the heat transfer recording system is required to accurately receive and fix the image ink fluidized by the thermal head. If the image-receiving sheet is an uncoated paper, the transferred image is liable to have white dropouts which deteriorate image quality, because the surface of such a paper is rough. Therefore, it is known that an image-receiving sheet coated with a coating composition consisting mainly of a pigment and an adhesive, the coated surface thereof being smoothed, can be used in place of the uncoated paper. Said image-receiving sheet coated with the coating composition has fewer white dropouts and better image quality than the uncoated paper. However, in said image-receiving coated sheet, the fixing ability of the transferred image tends to become worse. This causes image ink to be transferred reversely to the transferring sheet at the time of sheet release. As a result, the image density of the image-receiving coated sheet becomes low and partially irregular, and furthermore the transferred image is mottled.